As a part of the evolution of the Third Generation Partnership Project (3GPP) High Speed Packet Access (HSPA), both uplink transmit diversity (ULTD) and uplink multiple input, multiple output (UL MIMO) schemes are considered. Open-loop and closed-loop transmit diversity (referred to as OLTD and CLTD) are 3GPP Release-11 work items, whereas UL MIMO is a Release-11 study item. These schemes may be exploited to improve uplink spectral efficiency (peak-rate) and uplink coverage. In 3GPP RAN 50 meeting, CLTD was accepted as a work item. CLTD includes both closed-loop beam forming (CLBF) and closed-loop antenna switching (CLAS), but in general CLBF is considered to be of primary interest. In the same meeting, UL MIMO was proposed as a study item.
In order to improve system performance, a precoding technology is introduced into the above schemes. FIGS. 1 and 2 illustrate two user equipment (UE) structures for uplink closed-loop transmit diversity (UL CLTD) where such precoding technology is used.
In the UE structure 100 shown in FIG. 1, the Dedicated Physical Control Channels (DPCCHs) are precoded. The primary DPCCH (P-DPCCH) is precoded with the same precoding vector (often referred to as the primary precoding vector) as other channels such as the High Speed Dedicated Physical Control Channel (HS-DPCCH), the Enhanced Dedicated Physical Control Channel (E-DPCCH), and the Enhanced Dedicated Physical Data Channel (E-DPDCH). The secondary DPCCH (S-DPCCH) is precoded with another secondary precoding vector, which is preferably orthogonal to the primary precoding vector. Since the P-DPCCH is precoded with the same precoding matrix as that of the data stream, pilots associated with the P-DPCCH will see the same composite channel (also called effective channel) (i.e., spatial composite channel generated by precoding the raw channel) as that of for the data stream. With such a structure, existing defined physical channel power offsets relative to DPCCH may be reused.
FIG. 2 shows another UE structure 200 where the P-DPCCH and S-DPCCH are not precoded. In this case the existing definitions of physical channel power offsets relative to DPCCH should be updated. In 3GPP the current discussions seem to favor a precoded DPCCH structure in order to simplify the CLTD implementation. UL CLTD may be seen as a special case of UL MIMO. UEs in UL MIMO mode may benefit from improved bit-rates in the uplink. With MIMO, for example, up to two streams may be transmitted from different virtual antennas at the same time. FIGS. 3 and 4 provide two typical examples on how the two streams are transmitted in UL MIMO where precoding technology is used.
FIG. 3 shows an UE structure 300 where the P-DPCCH and S-DPCCH are precoded with the same precoding matrix as that of the data streams. A precoding matrix is often assumed to be unitary, having precoding vectors that are orthogonal to each other. FIG. 4 shows a UE structure 400 where the P-DPCCH and S-DPCCH are not precoded. Comparing the two structures 300, 400, for CLTD, precoding the DPCCHs with the same precoding matrix as used for the data streams simplifies the E-DCH power offset design and may facilitate a simpler soft handover mechanism.
Selection of a precoding matrix is involved in a precoding procedure, and a codebook-based precoding matrix selection mechanism is typically used. For UL CLTD and MIMO, when performing precoding, a precoding matrix used by the UE is usually selected by the serving NodeB from a codebook containing a plurality of precoding matrices, and this selected precoding matrix is indicated to the UE via precoding information such as precoding matrix indicator (PCI). The PCI is transmitted to the UE in the downlink (DL) so that the UE may determine a precoding matrix for precoding based on the PCI. The UE may also indicate the precoding matrix used by it to the NodeB by transmitting a PCI in the uplink (UL). Generally, for a structure with precoded DPCCH, this is not mandatory, since the NodeB may correctly decode a data stream without explicit information about the PCI. However, for a structure with non-precoded DPCCH, this is usually necessary because the non-serving NodeB needs PCI information to get a composite channel in order to decode the data stream. One common point for both precoded and non-precoded DPCCH structures is that PCI notification/feedback transmitted by the UE in the uplink may help the NodeB monitor a PCI transmission error in the downlink.
For both UL CLTD and MIMO, there is a possibility that the UE can receive a wrong PCI indication if a PCI transmission error occurs in the DL. Similarly, the NodeB may also receive a wrong PCI notification if a PCI transmission error occurs in the UL. Thus, the PCI received by the UE may not be the same as the PCI selected by NodeB, or the PCI received by the NodeB may not be the same as the PCI actually used by UE. In either case, the PCI assumed to be used by the UE as determined in the NodeB will be inconsistent with the PCI actually used by the UE. As a result, a mismatch between the precoding matrix assumed by the NodeB to be used by the UE and the precoding matrix actually used by the UE appears.
A PCI mismatch may affect at least several algorithms in the NodeB. In the scenario where the DPCCH is not precoded in UE, the NodeB requires a precoding matrix to decode received signals. A mismatched precoding matrix may result in decrease of decoding performance. In the scenario where the DPCCH is precoded in the UE, the NodeB requires a precoding matrix to perform raw channel estimation in order to select a precoding matrix for subsequent uplink transmission. A mismatched precoding matrix may result in wrong raw channel estimation and non-optimal precoding matrix selection. The same problem also exists in downlink MIMO. 3GPP simulations show that there is significant performance loss due to PCI transmission error.
Some schemes have been proposed to solve the above problems, where PCI/RI (rank indicator) is discarded when the downlink signal quality is worse than expected. In one scheme, if the estimated reception quality of present precoding information PCI sent from the NodeB is determined to be unreliable by the UE, the UE applies a pre-determined precoding matrix, or the most recently received PCI with good reception quality instead. Channel synthesis and filtering are also suggested to improve reception quality and channel estimation so as to increase the PCI transmission quality. However, none of these schemes solve the problems caused by PCI transmission errors.